So, I've been working on an audio pre-amp design for a while in my spare time. Since my wife and I just had a baby, I haven't had all that much spare time, but I thought I would outline my approach here to see if you all can keep me from making a(ny) stupid mistake(s). I have a nearly complete schematic, but I hesitate to post it, since going through a schematic is a lot to ask. I have a fair amount of experience, but not with audio, and not with mains-powered stuff.
The idea is to build a simple pre-amp with four stereo inputs, a volume control, and a balance control. Some other parameters are that there must not be any programmable parts (ROM's, uControllers, PLD's, etc.) and parts should not be too expensive or difficult to obtain. Ideally, everything would be stocked by Digikey. I would like the entire design to fit on a single circuit board with no wires running to any panel mount stuff.
If the design is any good at all, I'll post all the design info on the web somewhere so other people can build it if they want. I'm trying to avoid programmable parts to keep the entry barrier low. Plus, I don't have any kind of programmers at home, so it is convenient for me, too.
The power supply will use a standard AC receptacle (I'm in the US). I'll put a 5x20 mm fuse holder in series with the hot lead. Should I fuse neutral, too? There will be a mechanical pushbutton switch in series with the hot lead, also.
I'm using an 18 V 390 mA transformer (amvecco 70034, digikey TE70034-ND). I've got a bridge rectifier, and then an LM317 and an LM337 both in SOT223. I've selected resistors to generate +/- 15 V. There are also +/-
2.5V rails for logic. These are created from shunt regulators (TL431) from the +/- 15V.I don't need caps across the bridge for such a small power supply, right?
Also, is this the right Voltage secondary for a +/- 15 V supply?
I've got 220 uF 63V electrolytic capacitors after the bridge. Does that sound right?
Would you put a fuse on the secondary side for this kind of thing?
There are four stereo inputs and a volume control. There is one output. I am aiming for a max gain of 1V/V.
Should I provide more or is that reasonable for consumer CD and DVD sources and typical amplifiers? This is not critical since I can easily add gain later by changing resistors.
The inputs are buffered with OPA2134's in the non-inverting configuration. There is a shunt resistor (10k ?) to set the input impedance. After the OPA2134's, I go to a 74HC4052 analog mux which is followed by another OPA2134 which then goes to a digital pot and finally to another OPA2134.
This seems like way too many op-amps. My rationale is that I want to keep the 4052 sandwiched between a low impedance source and a high impedance load. Also, the digital pot I am using, the AD5220BR10, can't stand input Voltages higher or lower than its rails (+/- 2.5) So I am putting a string of four diodes to and from ground on the digital pot input to keep the Voltage from exceeding the rails. The digital pot cannot withstand +/-15 V rails, so connecting it up there is not an option. Because of this low clipping level, I am keeping the pot sandwiched between op-amps, too, so I can attenuate the signal before the pot, and amplify it back up after, if necessary.
Channel select is done via a counter which is clocked by a user pushbutton (with debounce and Schmitt trigger). So the user can't directly select a single channel, but has to go through them sequentially. But hey, there are only four channels.
Volume up/down and balance left right is done with pushbuttons, too. The balance logic increments the digital potentiometer on one channel and decrements the other. The volume just increments or decrements both channels.
There are a lot of things about this design I don't like. The biggest two issues are:
1) I don't like having +/- 2.5V rails for the logic (I'd rather just drive it from 5V to ground.) 2) There are too many op-amps.So, I am thinking of re-doing the volume. I could use the SSM2164. I would use the same potentiometers, followed by buffers to drive the control Voltage inputs on the SSM2164. I'd have to work out some way of doing balance, too, but I think that can be easily done so that one SSM2164 can do volume and balance for both channels. This would allow me to run from
+5V to gnd, instead of having split +/- 2.5V rails, get rid of the protection diodes, and lose a few op-amps in the signal path.The other option for driving the SSM2164 would be to use a free running clock connected to a counter, connected to a DAC. The DAC output would drive the control input on the SSM2164. The volume up pushbutton would enable the counter to count up, and volume down would enable it to count down. Balance would count up on one channel and down on the other. If this is a good idea, I'd love to have a short list of inexpensive DAC's to look at. I could also build a "DAC" using 1% resistors directly from the counter outputs. (You know, 8 resistors in series from the lsb, 7 from the next bit, ... up to 1 from the msb). Resistors are very cheap, and I don't need great accuracy, although monotonicity would be nice to have for volume, and I doubt I could rely on monotonicity with such a set-up. I guess I could just do the math and see how bad such a setup could be.
And there would have to be some kind of logic to keep the counters from wrapping around, since the user would find that non-intuitive. (The digital pot already does this internally.) I should be able to handle the logic, but if anybody has some recommendation for a counter that makes it particularly easy, that would be great.
Thanks for reading my longish post.
--Mac